1. Field of the Invention
This invention relates generally to large area semiconductor liquid junction devices suitable for use as photocells and in particular to such devices suitable for use as solar cells.
2. Description of the Prior Art
A desire exists for power supplies that would be both reasonably inexpensive and efficient as well as long-lived so as to permit the construction of large area primary power supplies that might be used for applications requiring, initially, power of the order of watts or kilowatts and, ultimately, even megawatts. Recent popular literature has contained the observation that the average integrated solar energy incident on a modest sized residence in middle latitudes could, with appropriate storage facilities and theoretical attainable efficiencies for solar energy conversion, result in an energy source sufficient to operate such a residence.
It is possible to establish certain directions that studies have taken of which at least one is likely to result in commercial success. It is reasonably clear, for example, from an economic viewpoint that much of the work conducted since the introduction of the practical power cell by Chapin, Fuller and Pearson, Journal of Applied Physics 25 676 (1954), while certainly of value for sophisticated uses such as the small power consumption electronic devices in satellites, would likely not lead directly to a large area commercially feasible power supply.
Studies have tended to establish a direction of likely significance; namely, the use of heterojunctions which permit near lossless transmission of light through a surface material defining one side of p-n junction, and also provide for essentially total absorption in the material on the side of the junction removed from the surface.
Other considerations have led to considerable work directed toward the use of polycrystalline material rather than single crystalline material in defining the junction. While techniques have been developed that result in significant improvement in the efficiency of polycrystalline materials, with appreciable lessening of trapping and other recombination sites that reduce the efficiency of photocurrent generation, even the best solid polycrystalline structures yet produced continue to evidence interfacial trapping and significantly lesser efficiencies than single crystal structures.
An approach companion to the preceding involves definition of a junction between a polycrystalline material and a liquid. A review paper describing this approach authored by Gerischer appeared in Electroanalytical Chemistry and Interfacial Electrochemistry 58 263-274 (1975). It was immediately apparent that this approach offered certain advantages in obviating problems arising in the preparation of heterojunctions between solid polycrystalline surfaces. It was clear that the substitution of a liquid for one of the solid layers avoided problems attendant upon lattice mismatch which, in all but cubic materials, was further complicated by crystalline directions.
Early devices of the liquid --solid type showed promise but were deficient in two respects: (1) efficiency (at a theoretical limit of approximately 20% to 25% assuming a solar spectrum corresponding to a black body emitter) was a level of one or a few percent; and (2) devices were quite unstable with lifetimes, as measured by the time the cell produced useful photocurrents, only of the order of minutes or at most several hours.
Further work took cognizance of the fact that chemical reactions between the redox couple and the polycrystalline electrode resulted in the removal or alteration of the surface of the latter and that instability of the cell was properly ascribed to the alteration or removal of the solid semiconductor with concomitant deterioration of the liquid-semiconductor junction.
Recognition of this problem led to the development of stable redox-solid semiconductor interfaces, e.g., by anodization within the redox couple itself, to produce a solid compound semiconductor which was expectedly stable in the very environment in which it was produced and subsequently operated. Further work, including that by Heller and coworkers Nature 262 680 (1976), improved the efficiency of cells using such interfaces to approximately the 7% level which is considered quite adequate for many contemplated purposes.
Structures of the type described, as well as variations also designed toward stable interfaces, resulted in elimination of chemical attack for many semiconductor materials and therefore in stabilization of the photocell against the cell degradation which characterized the early work of Gerischer and others. Nevertheless, it had been observed that while the initial efficiencies for such stabilized structures are quite reasonable, many of the cells deteriorated slowly but at a rate unacceptable for many design purposes.